count of unique values in pandas dataframe column

Question

I am trying to find the frequency of unique values in a column of a pandas dataframe I know how to get the unique values like this:

data_file.visiting_states()

returns :

array(['CA', 'VA', 'MT', nan, 'CO', 'CT'],    dtype=object)

and I want to return the count of those unique values and I know I cant do .value_counts() because its a numpy array

I cant do .value_counts() because its a numpy array, just cast it to series! pandas.Series(my_array).value_counts() — Paul H
– Paul H, Commented Jan 8, 2017 at 17:20

jezrael · Accepted Answer · 2017-01-08 16:05:46Z

1

You can use nunique:

data_file = pd.DataFrame({'visiting_states':['CA', 'VA', 'MT', np.nan, 'CO', 'CT','CA',
                                             'VA', 'MT', np.nan, 'CO', 'CT']})
print (data_file)
   visiting_states
0               CA
1               VA
2               MT
3              NaN
4               CO
5               CT
6               CA
7               VA
8               MT
9              NaN
10              CO
11              CT

print (data_file.visiting_states.nunique())
5

print (data_file.visiting_states.nunique(dropna=False))
6

arr = np.array(['CA', 'VA', 'MT', np.nan, 'CO', 'CT'],    dtype=object)
print (arr)
['CA' 'VA' 'MT' nan 'CO' 'CT']

print (len(arr))
6

answered Jan 8, 2017 at 16:05

jezrael

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4 Comments

dfjdlf Over a year ago

I want to know the count per state, ie 5 for CA, 4 for VA etc

jezrael Over a year ago

Then need value_counts - print (data_file.visiting_states.value_counts())

dfjdlf Over a year ago

this did not work, it said value counts is not defined

jezrael Over a year ago

ok, what return print (type(data_file.visiting_states)) and what print (type(data_file.visiting_states.iloc[0])) ?

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